vpe_run() is a weak symbol.  If there's no definition of it, its value is
zero.

If vpe_run is zero, return failure early.  We're going to fail anyway, so
there's no point in getting a VPE and attempting to load it.
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-mips@linux-mips.org
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10683/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>